Letter 13047

ZINC PLATING AND OLIVE BLACK PASSIVATION (CHROMATING) 

Hello everyone,

Can anybody clear my query as given below.

After Zinc plating & Olive black passivation on steel fasteners & fine blanked components (structural steel which are copper brazed with En8 material):

1. We face the problem on colour inconsistency - olive black not proper.
2. Also colour retentivity ( i.e. after few days olive black turns complete black).
3. Blister formation.
4. What should be the surface finish in the component.
5. What is the optimum plating thickness.
6. We require brake fluid compatibility( DOT3 & DOT 4 ).
7. What are the chemicals,their percentage etc. required to overcome the above problems.
8. How far the intricate components get affected compared to plane surfaces.
9. What is the expected life of components in NSS test.

Senthil Deepak Nandakumar
- Coimbatore, Tamil Nadu, India

Mr.Nandakumar,
This site is a place where peers exchange information and work together towards solving finishing problems. You've not yet helped anyone else with their inquiries, while your inquiry has 9 different broad-ranging requests. I mention this not as criticism, but to explain why inquiries like yours usually go unanswered. People just feel very little sense of obligation to help a person go into successful competition against them when that person presents himself as knowing nothing about metal finishing :-)

What proprietary olive black chromate or home brew formula do you use now? What kind of zinc plating do you do now--acid, alkali, or cyanide? What is the surface finish now? What thickness do you plate now? Do you have brake fluid compatability now? What thickness ratio do you get between the HCD and LCD areas now? What NSS life do you get now? Thanks.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

The plating system has two main parts to it: the main zinc plating, and the passivation. The thickness of the plating depends on your application. The auto parts we work with get around 8 microns of plating. The passivate is not specified by thickness.

Some of these technical issues should be addressed with your plating supplier. They should understand their processes well and should be able to educate you a bit on issues like this. You can also refer to the Metal Finishing Guidebook from Metal Finishing Magazine. This guidebook gives generic technical explanations of plating and of conversion coatings (passivation).

Sometimes is is very hard to produce an exact color consistantly. The color is dependant on the passivation, not necessarily the plating part. As you make part after part, the chemicals in the bath deplete, and the concentrations change over time. It can be difficult to maintain the passivation chemical bath the same all day every day. You may want to relax your color requirements to make things much easier for your plater. The plater may not have the capability to meet tight color requirements.

Tim Neveau
- Rochester Hills, MI

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